libnetlink: fix alignment of netlink messages

libnetlink: fix alignment of netlink messages

A padding to align a message should not only be added between
different attributes of a netlink message, but also at the end of the
message to pad it to the correct size.

Without this patch the following command does not work and returns an
error code:
ip link add type nlmon

Without this ip from busybox sends this:

sendmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000},
	msg_namelen=12, msg_iov=[{iov_base={{len=45, ...},
		"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\20\0\22\0\t\0\1nlmon"}, iov_len=45}],
	msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 45
return value: 2

The normal ip utile from iproute2 sends this:

sendmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000},
	msg_namelen=12, msg_iov=[{iov_base={{len=48, ...},
		"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\20\0\22\0\t\0\1nlmon\0\0\0"}, iov_len=48}],
	msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 48
return value: 0

With this patch ip from busybox sends this:

sendmsg(3, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000},
	msg_namelen=12, msg_iov=[{iov_base={{len=48, ...},
		"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\20\0\22\0\t\0\1nlmon\0\0\0"}, iov_len=48}],
	msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 48
return value: 0

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>